During the drilling of an oil and gas well, a specialized fluid referred to as a drilling fluid or alternatively a “mud” is circulated through the drill pipe and bit. The principal functions of a drilling fluid include: stabilizing geological formations, providing hydrostatic pressure, cooling the drill bit, and carrying drill cuttings beneath the bit to transport them up to the surface for separation. The other key function of a drilling fluid is to act a lubricant between the drill pipe and the borehole and/or metal casing. The drilling fluid also acts as a lubricant for the drill bit.
Drilling fluids can be categorized as being either water-based or oil-based. In general, water-based drilling fluids are lower cost and have much better health, safety and environmental performance than oil-based drilling fluids. However, oil-based fluids offer certain performance advantages such as excellent shale stabilization. Sodium & potassium silicate based drilling fluids are one of the few water-based drilling fluids that can match the shale inhibition properties of oil based drilling fluids. The efficacy of silicate-based systems for shale inhibition has been known since the 1930's. The shale inhibition properties of silicate-based drilling fluids have been well documented in numerous scientific documents such as, for example, E. van Oort, Shell Research Rijswijk; D. Ripley, I. Ward, J. W. Chapman, BW Mud Aberdeen, R. Williamson Mobil, M. Aston, BP Exploration: Silicate-Based Drilling Fluids: Competent, Cost-effective and Benign Solutions to Wellbore Stability Problems”, SPE paper 35059 presented at SPE/IADC Drilling Conference, New Orleans, La., March 1996.
Silicate-based drilling fluids began to make a commercial comeback in the 1990's beginning in the North Sea. The resurgence of silicate-based drilling fluids was driven by a combination of favorable economics, demand for products with better environmental performance and advancements in drilling fluid formulation technology (e.g. synthetic polymers for rheology control). Silicate-based drilling fluids have continued to gain in commercial acceptance and are used around the globe for offshore and on-land drilling operations. It is anticipated that this trend will continue as Regulatory Bodies as well as the public place greater scrutiny on the drilling and completion of wells. The environmental performance of silicate-based fluids is also well documented, such as in Duncan, Petro Canada, M. McDonald, National Silicates: “Exceeding Drilling Performance and Environmental Requirements with Potassium Silicate Based Drilling Fluid”, SPE 86700, International Conference on HS&E, Calgary, Alberta, March 2004.
Over the decades, the vast majority of drilling fluids have been formulated with either sodium silicate or potassium silicate using standard, commercially available ratio products. However, such drilling fluids have not used high ratio aqueous alkali silicates. Other forms of alkali silicate do exist and it is anticipated that those forms of alkali silicate would also show similar benefits with higher ratio as presented below with respect to sodium silicate and potassium silicate.
In the silicate industry, the term ratio typically refers to the weight ratio of SiO2 to Me2O (where Me is the alkali and is most commonly sodium or potassium). Among scientists, it is more common to refer to ratio as the molar ratio of SiO2 to Me2O. Coincidentally, the molecular weight of Na2O (62) and SiO2 (60) are close enough that the molar ratio and weight ratio can be used interchangeably for sodium silicate. For other sources of alkali silicate, the weight ratio does not match the molar ratio. Reference will be made herein to specify whether the ratio refers to weight or molar ratio.
Table I below, which is derived from U.S. Pat. No. 5,624,651 to Bass, presents the composition and typical properties of commercial grades of liquid sodium silicate and potassium silicate.
TABLE IMolarAlkaliWt. RatioRatioSiO2Na2ODensityViscosityMetalSiO2:M2OSiO2:M2O(%)(%)(lb/gal)(centipoise)Sodium3.753.8725.36.7511.02203.253.3629.99.2211.88303.253.3628.48.711.61603.223.3327.78.611.51002.872.9732.011.112.41,2502.582.6732.112.512.67802.502.5826.510.611.7602.402.4833.213.8513.02,1002.202.2729.213.312.5—2.002.0729.414.712.84002.002.0736.018.014.170,0001.901.9628.515.012.7—1.801.8624.113.412.0601.601.6531.519.714.07,000Potassium2.503.9220.88.310.5402.203.4519.99.0510.572.103.2926.312.511.51,050
Ratio is a major parameter that determines the type of silicate species in solution. Silicate speciation refers to the size and shape of silicate molecules found in solution. The building block for these silicate species is the SiO4 monomer. FIG. 1 shows a small sample of the various silicate species that can be found in a silicate solution (e.g., monomer, dimers, trimers, oligomers, chains, rings of silicate anions, etc.).
FIG. 2 shows the trend towards high molecular weight, more complex polysilicate anions with increasing ratio of SiO2:Me2O.
Because of differences in the size, shape and distribution of silicate species, there will be different rates of chemical reactions and varying degrees of interactions in the drilling fluids. It has been discovered the larger, more complex polysilicate anions found in aqueous high ratio silicate can be particularly beneficial for drilling conventional and unconventional hydrocarbons. A drilling fluid can be formulated with these larger more complex polysilicate anions by increasing the ratio of SiO2:Me2O beyond the standard, commercially available aqueous alkali silicates.
The ratio of SiO2:Me2O is not increased to the point where the alkali silicate could be considered a silica sol also often referred to as colloidal silica. Sols are stable dispersions of discrete amorphous silica particles in a liquid, usually water. Commercial products contain silica particles having a diameter of about 3-100 nm, specific surface areas of 50-270 m2/g and silica contents of 15-50 wt %. According to Kirk-Othmer Encycolpedia of Chemical Technology, Fourth Edition, Volume 21, ISBN 0-471-52690-8, Copyright 1997 by John Wiley & Sons, such silica sols contain small (<1 wt %) amounts of stabilizers, most commonly sodium ions. A silica gel is a three dimensional network of silica particles.
Alkali silicate-based drilling fluids stabilize shales by the chemical reaction of the silicates on the shale surface and in the shale pores. Upon entering the shale pore network the silicate anions react with multivalent cations (e.g. Ca+2, Mg+2, Al+3, Fe+3, etc) present in the pore fluid and form an insoluble silicate metal precipitate. A second reaction is the pH of the pore fluid is neutral which causes the silicate anions to self-polymerize into a silica gel. These reactions are discussed in E. van Oort, Shell Research Rijswijk; D. Ripley, I. Ward, J. W. Chapman, BW Mud Aberdeen, R. Williamson Mobil, M. Aston, BP Exploration: Silicate-Based Drilling Fluids: Competent, Cost-effective and Benign Solutions to Wellbore Stability Problems”, SPE paper 35059 presented at SPE/IADC Drilling Conference, New Orleans, La., March 1996. It is also noted in the same paper that shales containing small fractures are destabilized by drilling fluids that invade the cracks and elevate the fluid pressure in them. Soluble silicates have the ability to fill and pressure seal microfractures. The mechanism is again based on the gelling and precipitation of the silicate anions.
The polysilicate anions present in aqueous high ratio alkali silicates have a lower buffering capacity than conventional ratio silicates and therefore can more easily undergo the polymerization reaction. The polysilicate anions also require a lower concentration of available multivalent metals to form an insoluble metal silicate. High ratio silicate are therefore ideally suited for drilling that requires traditional prevention of shale hydration as well as stabilization by sealing microfractures and the prevention of shale delamination. These drilling challenges are commonly seen in the drilling of shale gas. High ratio aqueous alkali silicates are also suitable for stabilizing and mitigating drilling fluid loss in highly permeable and/or depleted zones.
Tar sands represent another type of unconventional hydrocarbons that can be drilled using an aqueous high ratio, aqueous alkali silicates as a drilling fluid additive. A commonly used method for producing tar sands is SAGD (steam assisted gravity drainage). The SAGD process involves drilling two parallel wells. The wells are drilled vertically through shale and an angle is then built as the well approaches the bitumen section. The bitumen section is drilled horizontal. The top shale sections often require inhibition. Upon drilling the bitumen, a key performance criteria requirement of a SAGD drilling fluid is the prevention of accretion. That is, bitumen sticking to metal surfaces such as the drill pipe. The drilling fluid should also provide a wellbore surface that is receptive to cement. A competent cement bond in the top and build section is required to isolate injected steam and prevent the leakage of gas. The aqueous high ratio alkali silicates were shown to prevent accretion as well as provide a suitable wellbore surface for a superior cement bond.
The higher level of alkalinity inherent in standard ratio aqueous sodium silicate and potassium silicate products can have drawbacks. This is particularly true when looking at the health, safety and environmental characteristics of silicate-based drilling fluids. Silicate-based drilling fluids are considered moderately alkaline and this alkalinity may cause minor skin and eye irritation. Factors affecting irritation include silicate concentration, exposure time and the ratio of the aqueous alkali silicate. Higher ratio aqueous sodium and potassium silicate allow for reduced alkalinity versus standard ratio aqueous silicates at similar concentrations. It also allows for the formulation of drilling fluids with higher concentrations of aqueous silicate with reduced risks associated with alkalinity.
Alkalinity also impacts the disposal of silicate-based drilling waste. At well completion there is a large volume of spent drilling fluid and fluid covered drill cuttings that need to be properly disposed. The toxicity and environmental performance of the drilling fluid and local government regulations dictate the disposal method for the drilling waste. In the case of drilling waste being disposed on soil surfaces, regulators usually have restrictions on drilling waste salinity. Salinity is typically measured by using the criteria of;                electrical conductivity (EC)        sodium adsorption ratio (SAR):        
  SAR  =            [              Na        ⁢                  +                    ]                                1          2                ⁢                  (                                    [                              Ca                +                2                            ]                        +                          [                              Mg                +                2                            ]                                )                                    sodium content        
Higher ratio silicates also show a reduction in some or all of these salinity values.
Prior art aqueous alkali silicate drilling fluids have been made with commercially available material. U.S. Pat. No. 2,133,759 to Vail et al. describes a silicate drilling fluid. Vail et al. notes that the composition of solid silicate gives desirable results over a wide range. The patent describes a silicate of soda having a ratio of SiO2 to Na2O ranging from about 1.5: to 4:1.
U.S. Pat. No. 2,146,693 to Vietti et al. proposes to use “commercially available” solutions of sodium silicate. Vietti et al. describes sodium silicate with a molecular ratio of silica to sodium oxide greater than 1 and preferably in the range of 1.1:1 to 3.9:1. A ratio higher than 4.0 would not be commercially available.
U.S. Pat. No. 4,988,450 to Wingrave relates to an environmentally safe shale stabilization additive in an aqueous drilling fluid. Wingrave notes that potassium silicates can be obtained commercially in a variety of SiO2 to K2O ratios. Those silicates having a SiO2/K2O ratio in the range of about 0.5 to 2.5 are suitable for use in the present invention, however, the range of about 1.5 and 2.2 is preferred
More recently, U.S. Pat. Nos. 7,137,459 and 7,441,609 to Dearing address some of the issues that sodium and potassium silicate can have on drilling fluid. These include lack of compatibility with commonly used lubricants for the reduction of the Coefficient of Friction in the drilling fluid as well as reacting with accumulated solids in the drilling fluid to create control problems for certain drilling fluid properties.
Dearing '459 uses a novel method for formulating a silicate-based drilling fluid. Potassium or sodium silicate as a solid glass is continuously added into the flow line as the drilling fluid is pumped downhole. The rate of addition of silicate ideally matches the rate of depletion on the wellbore. The patent claims adding a solid alkali metal silicate glass to the well flow line at a soluble silica concentration from 0.1 to 0.25% by weight. Tables A and B in Dearing '459 set forth the range of SiO2:Me2O ratio silicates glasses available commercially. Dearing '429 does extend beyond the standard ratios to include a suitable weight ratio range weight range of SiO2:K2O from 0.5:1 to about 3:1 For sodium silicate a weight range of SiO2:Na2O from 1:1 to about. 4:1. The alkali silicate glass is added at a sufficient rate to dissolve and react with the drilled cuttings and freshly exposed surface of the borehole while leaving very little excess soluble silicate in solution.
Dearing '609 expands on the use of finely ground, very sparingly soluble, anhydrous alkali metal silicate glass material to decrease the swelling and water sorption of shale. The patent states that although sodium and potassium silicates have been used for decades to combat shale problems they have had limited success due to free soluble silicate in the drilling fluid. The main technical benefit of finely ground sparing soluble alkali metal silicate glass instead of aqueous liquid silicates is the slower rate of dissolution will reduce undesirable reactions of the silicate. The patent claims potassium silicate and the potassium silicate has a weight ratio of SiO2:K2O from 2:1 to about 3.5:1. The patent claim a concentration of 0.1% to about 1% by weight soluble silica in the aqueous phase of the drilling fluid. As noted in Dearing '609 the use of a solid contrasts with liquid alkali silicates.
Silica and silicate chemistry dictates that these material go into solution as a monomeric silicic acid (Si(OH)4) or the silicate ion (HSiO3−) depending on the pH. Once in solution, silicate speciation is strongly affected by other factors including concentration of silicate in solution, source of silica, temperature and time. On a molecular level, sparing soluble potassium silicate would be going into solution as the monosilicate anion. Once in solution it would quickly react with drill cuttings and exposed surfaces of the borehole. This contrasts with this inventions, where the silicate exists as large, complex polysilicate anions.
The sparingly soluble glass with the resulting low concentration of soluble silicate and the lower molecular weight silica species is useful but has limits for wellbore stabilization.
More recently, nanotechnology has been proposed as a method of shale stabilization. In United States Published Patent Application No. 2009/0314549 A1, Chenevert proposes various nanoparticles including silica nanoparticles to reduce shale permeability by using the nanoparticles to mechanically plug shale pore throats. The polysilicate anions in high ratio aqueous alkali silicates are single-phase soluble chemistries, not solid separate phases of silica dispersed in water. Similar to standard ratio alkali silicates, the high ratio aqueous alkali silicates achieves shale stabilization by chemically reacting with the shale. Beyond preventing shale hydration, high ratio aqueous alkali silicates provide other forms of wellbore stabilization such as sealing microfractures and preventing shale delamination.
As seen in the prior art, soluble silicates can be added to a drilling over a very wide range of concentration to achieve the desired level of wellbore stability. High ratio aqueous alkali silicates can likewise be formulated over a wide range of concentrations to match the inhibition requirements of the shale being drilled.